Known systems for recovery of mechanical vibration energy generally comprise a base rigidly fixed to a moving support, a mobile part making a relative movement due to its inertia with regard to this fixed part, a flexible link enabling relative displacement between the fixed and mobile parts and a converter for transforming mechanical energy from the mobile part to retrieve it in a different form (for example electrical).
In the case of an electrostatic conversion structure like that described in document US 2005093302 A1, energy is converted through a variable capacitor. This capacitor is formed from electrodes connected to the fixed vibrating support and electrodes connected to the mobile mass. The relative movement of these electrodes causes a variation in the value of the capacitance of the capacitor. This variation in capacitance is then used to transform the energy of the mechanical movement into electrical energy.
Electrostatic conversion structures may be classified in three categories.                A structure comprising one fixed electrode and one mobile electrode, the electrodes being arranged in two parallel planes, the mobile electrode being arranged at a distance from the fixed electrode and moving away from and towards the fixed electrode due to the vibrations.        A structure comprising a fixed electrode provided with fingers, a mobile mass provided with fingers, the fingers of the fixed electrode and the fingers of the mobile mass being interdigitized. During movement of the mobile mass in the direction of the fingers, the overlap between the fingers of the fixed electrode and the mobile mass varies in a plane.        For the third family, the structure is similar to the previous structure, however the mobile mass is moved perpendicular to the direction of the fingers.        
The distance separating the fixed electrode and the mobile electrode at rest is called the air gap distance. This distance is fixed for each system described above. Thus, the displacement amplitude of the mobile electrode with respect to the fixed electrode is invariable, and is determined when the system is manufactured. The maximum allowable vibration amplitude in each case corresponds to the value of the air gap at rest between the fixed part and the mobile part.
Structures in known energy conversion systems are designed to be perfectly adapted to a well defined environment in terms of amplitude, vibration and deformation. Consequently, these structures become less efficient or even unusable for environments in which these parameters can vary with time.
The electrical energy retrieved during displacement of the mobile part depends on the variation in the capacitance that occurs during this displacement. For example, in the case of operation at constant charge on the variable capacitor, the energy that can theoretically be recovered for each recovery cycle is given by:E=½UchargeUdischarge(Cmax−Cmin),                Ucharge and Udischarge being the charge and discharge voltages at the capacitor terminals, and        Cmax−Cmin represents the maximum variation of the capacitance of the capacitor.        
This energy is directly proportional to voltages at which the charge is injected and recovered on the capacitor, and the variation in the capacitance obtained during this movement.
Laws on electrostatic conversion structures giving the value of the capacitance as a function of the relative position X of the mobile part and the fixed part are of the type 1/(X−Δ) and 1/(X−Δ)2, in which Δ is the value of the air gap distance at rest. Therefore, the capacitance is minimum at rest when X is zero, and maximum when the movement amplitude moves towards the value of the air gap distance.
It is then observed that to maximize the recovered energy, it is preferable for the fixed and mobile electrodes to move as close as possible towards each other, which maximizes the maximum capacitance Cmax and therefore the final variation of the capacitance Cmax−Cmin.
Known types of conversion systems can only operate optimally with vibration sources that are stable in amplitude and/or frequency. These vibration sources rarely satisfy these stability conditions. The result is that the value of the maximum capacitance Cmax cannot be maximized at all times. FIGS. 6A and 6B show the variation of the variable capacitance as a function of a sinusoidal excitation with decreasing amplitude, for a known type of conversion structure in the plane. It is observed that the value of the maximum capacitance Cmax reduces very quickly with the vibration amplitude: a variation in the vibration amplitude of a few microns, for example when changing from an amplitude of 25 μm to an amplitude of 18 μm, results in a total capacitance variation changing from 170 pF to 70 pF. The recoverable energy is then divided by a factor of almost 3.
Consequently, one of the purposes of this invention is to propose a system for conversion of vibrational mechanical energy into electrical energy that can operate in an optimum manner, particularly with vibration sources with a variable amplitude.